French troops in Chad prepare to be airlifted into Mali yesterday. Photograph: Nicolas Vissac/AFP/Getty Images

Britain announced on Saturday night that it was deploying aircraft to assist French military operations against Islamist rebels in Mali as an escalation in hostilities was claimed to have killed more than 120 people.

David Cameron's offer to transport foreign troops and equipment involved Britain in a fresh conflict that could provoke terrorist reprisals against European targets. President François Hollande yesterday placed France on high alert as French planes bombarded targets in Mali.

Downing Street said two transport planes would be dispatched, but British troops would not join the French military mission to help recapture the north of Mali from al-Qaida-linked rebels acting against the country's government.

"The prime minister spoke to President Hollande this evening to discuss the deteriorating situation in Mali and how the UK can support French military assistance provided to the Malian government to contain rebel and extremist groups in the north of the country," a spokeswoman said. "Both leaders agreed that the situation in Mali poses a real threat to international security given terrorist activity there."

Earlier, Hollande warned that two days of air strikes by French war planes were only the opening salvoes in a longer campaign. "We have already held back the progress of our adversaries and inflicted heavy losses on them. But our mission is not over yet," he said.

The latest aerial bombardment led to the death of a French pilot, Damien Boiteux, and, according to a senior army officer in Mali, those of more than 100 rebel troops following fighting for the strategic town of Konna. Malian officials said 11 government soldiers had been killed in efforts to wrest the town from rebel control. Human rights groups counted 10 civilian deaths.

France insists it is undertaking military operations in Mali, which had been a stable democracy until a military coup last March paved the way for the Islamist rebellion, to provide support to a West African troop deployment backed by the United Nations.

Regional economic bloc Ecowas has accelerated its efforts to send troops to the international campaign in Mali, authorising the immediate deployment of 3,300 troops.

The United States was also said to be weighing possible involvement with the Pentagon considering options such as intelligence-sharing with France and logistics support.

For Hollande, the intervention in Mali represents the biggest foreign policy test he has faced since becoming president in May. So far he has enjoyed widespread political support at home and abroad for the African mission.

France's defence minister, Jean-Yves Le Drian, said hundreds of French troops and aircraft had been involved in fighting at three locations in the centre of Mali, including against an Islamist command centre.

A French army unit also attacked a column of rebels heading towards the town of Mopti. He insisted that France was compelled to act quickly to stop the Islamist offensive, which he said could allow "a terrorist state at the doorstep of France and Europe".

In a separate military operation in Somalia, a French soldier was killed during a botched commando raid on an Islamist compound to rescue a captured secret service agent. The hostage is also believed to have been killed in the operation.

Another commando is listed as missing amid claims that he was injured and captured by fighters belonging to the Islamist al-Shabaab movement.

The operation had failed "despite the sacrifice of two of our soldiers and without doubt the assassination of our hostage", Hollande said. But he said it confirmed "France's determination not to give in to the blackmail of terrorists" and reiterated his commitment to pursuing military intervention in Mali.

Although officials denied there was any connection between the rescue effort and the operation launched in Mali, the French military escalation would have complicated the position of the hostage in Somalia.

The secret service agent, "Denis Allex", is believed to have been killed by his captors during a failed helicopter raid in Bula Mareer, 70 miles south of Mogadishu. The assault faltered after resistance at the compound, which was reinforced by fighters at a neighbouring training camp who heard the helicopters.

The agent and a colleague were kidnapped in 2009 while assigned to the international effort to assist Somalia's transitional government in Mogadishu. His colleague escaped a month later.

Residents of the town described explosions and gunfire while an al-Shabaab official said that the fighting began after helicopters had dropped off French commandos.

The French ministry of defence said that the decision to launch the raid to rescue Allex had been taken after there had been no progress in three years of attempted negotiations to secure his release.

"Faced with the intransigence of the terrorists, who refused for three years to engage in all negotiations, and who were holding Denis Allex in inhuman conditions, an operation was planned and set in effect," said a spokesman.